The present invention relates to a method in which a cationic electrodeposition paint and a water based intermediate coat paint are applied by wet-on-wet and then heated to cross-link and cure both paint films at the same time, that is, coating is carried out by a so-called xe2x80x9c2 coat 1 bakexe2x80x9d system (2C1B) to form a multi-layer paint film, particularly to a method for forming a multi-layer paint film which is improved in a curing property, a chipping resistance, a corrosion resistance, a finishing appearance (smoothness, gloss feeling and the like) and an interlayer adhesive property between both paint films.
It is known to apply a cationic electrodeposition paint containing a block polyisocyanate compound as a cross-linking agent and a water based intermediate coat paint containing an amino resin as a cross-linking agent in order on a conductive coated article such as an outside plate of a car body by wet-on-wet and then heat and cure both paint films at the same time by heating to form a multi-layer paint film. However, this multi-layer paint film does not have a satisfactory finishing appearance such as smoothness, gloss feeling and the like, and it is difficult to solve this defect even by further applying a top coat paint. Further, automobiles using an outside plate on which such multi-layer paint film is formed have the problem that when small stones sent flying during running hit against such multi-layer paint film, chipping peeling is liable to be caused in a layer between both paint films.
An object of the present invention is to solve the problems described above in a method for applying a cationic electrodeposition paint and a water based intermediate coat paint by 2C1B to form a multi-layer paint film.
Intensive researches repeated by the present inventors have resulted in finding that the object described above can be achieved by using a block polyisocyanate compound as a cross-linking agent for both cationic electrodeposition paint and water based intermediate coat paint and adding a specific amount of a tin base catalyst to the cationic electrodeposition paint, and they have come to complete the present invention.
Thus, according to the present invention, provided is a method for forming a multi-layer paint film, comprising:
applying a cationic electrodeposition paint (A) containing a block polyisocyanate compound as a cross-linking agent on a surface of a substrate,
applying a water based intermediate coat paint (B) containing a block polyisocyanate compound as a cross-linking agent on an electrodeposition paint film surface without curing the electrodeposition paint film to form an intermediate coat paint film, and then
heating to cure both of the electrodeposition paint film and the intermediate coat paint film at the same time to thereby form a multi-layer paint film,
wherein the above cationic electrodeposition paint (A) contains a tin base catalyst of at least 10 parts by weight per 100 parts by weight of the resin solid matter, and a cross-linking curing reaction in the electrodeposition paint film is controlled so that it starts earlier than a cross-linking curing reaction in the intermediate coat paint film.
In the present invention, a cross-linking curing-starting time in the paint films of the cationic electrodeposition paint (A) and the water based intermediate coat paint (B) can be determined by means of a pendulum type visco-elasticity measuring device (LEOVI-BRON DDV-OPA type, manufactured by Toyo Boldwin Co., Ltd.). To be specific, a pendulum having a weight of 22 g and an inertia moment of 850 gxc2x7cm2 is used, and this pendulum is put on an uncured paint film of a paint applied on a steel plate so that the cured paint film has a film thickness of 30 xcexcm. The above paint film is heated at a prescribed temperature (for example, 140 to 180xc2x0 C.) for cross-linking and curing the paint film while vibrating the pendulum, and the time when a value of a logarithmic decrement of the pendulum starts going up is referred to as xe2x80x9ccross-linking curing-starting timexe2x80x9d. In this case, the time spent from commencement of heating up to cross-linking curing-starting time is referred to as xe2x80x9ccuring-starting timexe2x80x9d, and the shorter time thereof means that xe2x80x9ca cross-linking curing reaction starts earlierxe2x80x9d. The cross-linking curing-starting times of both paint films of the cationic electrodeposition paint (A) and the water based intermediate coat paint (B) are compared on the basis of results obtained by measuring at he same heating temperature.